Optical pickup device

ABSTRACT

An optical pickup device has an astigmatism element which imparts astigmatism to reflected light of laser light reflected on a recording layer, and a spectral element into which the reflected light is entered, and which separates the reflected light. The spectral element is divided into six second areas by a straight line in parallel to a first direction, a straight line in parallel to a second direction, and a first area having a predetermined width and formed along a straight line in parallel to a third direction inclined from the first direction by 45 degrees. The spectral element is configured to guide the reflected light passing through the six second areas to corresponding sensors on a photodetector while making propagating directions of the reflected light different from each other, and to avoid guiding the reflected light entered into the first area to the sensors.

This application claims priority under 35 U.S.C. Section 119 of JapanesePatent Application No. 2010-172210 filed Jul. 30, 2010, entitled“OPTICAL PICKUP DEVICE”. The disclosure of the above application isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The invention relates to an optical pickup device, and more particularlyto an optical pickup device suitable for use in irradiating a recordingmedium having plural laminated recording layers with laser light.

2. Disclosure of Related Art

In recent years, as the capacity of an optical disc has been increased,an optical disc having an increased number of recording layers has beendeveloped. Laminating recording layers in a disc enables to considerablyincrease the data capacity of the disc. In the case where recordinglayers are laminated, generally, two recording layers are laminated onone side of a disc. Recently, however, laminating three or morerecording layers on one side of a disc has been put into practice tofurther increase the capacity of the disc. Thus, the capacity of a disccan be increased by increasing the number of recording layers to belaminated. However, as the number of recording layers to be laminated isincreased, the distance between the recording layers is decreased, andsignal deterioration resulting from an interlayer crosstalk isincreased.

As the number of recording layers to be laminated is increased,reflection light from a recording layer (a targeted recording layer) tobe recorded/reproduced is reduced. As a result, if unwanted reflectionlight (stray light) is entered into a photodetector from a recordinglayer on or under the targeted recording layer, a detection signal maybe deteriorated, which may adversely affect focus servo control andtracking servo control. In view of this, in the case where a largenumber of recording layers are laminated, it is necessary to properlyremove stray light, and stabilize a signal from a photodetector.

Japanese Unexamined Patent Publication No. 2009-211770 (corresponding toU.S. Patent Application Publication No. US2009/0225645 A1) discloses anovel arrangement of an optical pickup device operable to properlyremove stray light, in the case where a large number of recording layersare formed. With this arrangement, it is possible to form an area whereonly signal light exists, on a light receiving surface of aphotodetector. By disposing a sensor of the photodetector in the abovearea, it is possible to suppress an influence on a detection signalresulting from stray light.

In the above optical pickup device, an area onto which signal light isirradiated, and an area onto which stray light is irradiated areadjacent to each other. As a result, even if a sensor is disposed in anarea where only signal light exists, apart of stray light may be enteredinto the sensor, which may degrade the precision of a detection signal.Further, if the position of the sensor disposed on the photodetector isdisplaced in the above optical pickup device, a detection signal may bedegraded depending on positional displacement amount.

SUMMARY OF THE INVENTION

A main aspect of the invention relates to an optical pickup device. Theoptical pickup device according to the main aspect includes a lightsource which emits laser light; an objective lens which focuses thelaser light on a recording layer; an astigmatism element which impartsastigmatism to reflected light of the laser light reflected on therecording layer; a spectral element into which the reflected light isentered, and which separates the reflected light; and a photodetectorwhich receives the reflected light. In the above arrangement, theastigmatism element converges the reflected light in a first directionand in a second direction perpendicular to the first direction so thatthe reflected light forms focal lines at different positions from eachother. The spectral element is divided into six second areas by astraight line in parallel to the first direction, a straight line inparallel to the second direction, and a first area having apredetermined width and formed along a straight line in parallel to athird direction inclined from the first direction by 45 degrees. Thespectral element is configured to guide the reflected light passingthrough the six second areas to corresponding sensors on thephotodetector while making propagating directions of the reflected lightdifferent from each other, and to avoid guiding the reflected lightentered into the first area to the sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, and novel features of the present inventionwill become more apparent upon reading the following detaileddescription of the embodiment along with the accompanying drawings.

FIGS. 1A and 1B are diagrams for describing a technical principle (as tohow light rays converge) in an embodiment of the invention.

FIGS. 2A through 2D are diagrams for describing the technical principle(as to how light fluxes are distributed) in the embodiment.

FIGS. 3A through 3D are diagrams for describing the technical principle(as to how signal light and stray light are distributed) in theembodiment.

FIGS. 4A and 4B are diagrams for describing the technical principle (amethod for separating light fluxes) in the embodiment.

FIGS. 5A through 5D are diagrams for describing a method for arrangingsensors in the embodiment.

FIG. 6 is a diagram showing a preferable range to which the technicalprinciple of the embodiment is applied.

FIGS. 7A through 7D are schematic diagrams showing an arrangement of aspectral element based on the technical principle of the embodiment, andan irradiation area in the case where the spectral element is used.

FIGS. 8A through 8F are diagrams for describing an output signal fromeach sensor resulting from positional displacement of a sensor, in thecase where the spectral element based on the technical principle of theembodiment is used.

FIG. 9 is a diagram for describing an approach of suppressing an offset(DC component) of a push-pull signal resulting from positionaldisplacement of a sensor, in the case where the spectral element basedon the technical principle of the embodiment is used.

FIGS. 10A through 10C are diagrams showing an optical system of anoptical pickup device in an inventive example.

FIGS. 11A and 11B are diagrams showing an arrangement of a spectralelement in the inventive example.

FIG. 12 is a diagram showing a sensor layout of a photodetector in theinventive example.

FIGS. 13A through 13C are schematic diagrams showing irradiation areasin the inventive example.

FIGS. 14A through 14F are diagrams for describing an output signal fromeach sensor resulting from positional displacement of a sensor in aninventive example.

FIGS. 15A through 15D are diagrams showing a simulation result, in thecase where the spectral element based on the technical principle of theembodiment is used, and in the case where the spectral element in theinventive example is used.

FIGS. 16A through 16D are diagrams showing a simulation result, in thecase where the spectral element based on the technical principle of theembodiment is used, and in the case where the spectral element in theinventive example is used.

FIGS. 17A and 17B are diagrams showing a modification example of thearrangement of the spectral element in the inventive example.

FIGS. 18A and 18B are diagrams showing a modification example of thearrangement of the spectral element in the inventive example.

FIGS. 19A and 19B are diagrams showing a modification example of thearrangement of the spectral element in the inventive example.

The drawings are provided mainly for describing the present invention,and do not limit the scope of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, an embodiment of the invention is described referringto the drawings.

Technical Principle

First, a technical principle to which the embodiment of the invention isapplied is described referring to FIGS. 1A through 6.

FIG. 1A is a diagram showing a state as to how light rays are converged.FIG. 1A is a diagram showing a state as to how laser light (signallight) reflected on a target recording layer, laser light (stray light1) reflected on a layer located at a rearward position with respect tothe target recording layer, and laser light (stray light 2) reflected ona layer located at a forward position with respect to the targetrecording layer are converged. FIG. 1B is a diagram showing anarrangement of an anamorphic lens to be used in the technical principle.

Referring to FIG. 1B, the anamorphic lens has a function of converginglaser light to be entered in a direction in parallel to the lens opticalaxis, in a curved surface direction and a flat surface direction. Thecurved surface direction and the flat surface direction intersectperpendicularly to each other. Further, the curved surface direction hasa smaller radius of curvature than that of the flat surface direction,and has a greater effect of converging laser light to be entered intothe anamorphic lens.

To simplify the description on the astigmatism function of theanamorphic lens, the terms “curved surface direction” and “flat surfacedirection” are used. Actually, however, as far as the anamorphic lenshas a function of forming focal lines at different positions from eachother, the shape of the anamorphic lens in the “flat surface direction”in FIG. 1B is not limited to a flat plane shape. In the case where laserlight is entered into the anamorphic lens in a convergence state, theshape of the anamorphic lens in the “flat surface direction” may be astraight line shape (where the radius of curvature=∞).

Referring to FIG. 1A, signal light converged by the anamorphic lensforms focal lines at different positions from each other by convergencein the curved surface direction and in the flat surface direction. Thefocal line position (S1) of signal light by convergence in the curvedsurface direction is close to the anamorphic lens than the focal lineposition (S2) of signal light by convergence in the flat surfacedirection, and the convergence position (S0) of signal light is anintermediate position between the focal line positions (S1) and (S2) byconvergence in the curved surface direction and in the flat surfacedirection.

Similarly to the above, the focal line position (M11) of stray light 1converged by the anamorphic lens by convergence in the curved surfacedirection is close to the anamorphic lens than the focal line position(M12) of stray light 1 by convergence in the flat surface direction. Theanamorphic lens is designed to make the focal line position (M12) ofstray light 1 by convergence in the flat surface direction close to theanamorphic lens than the focal line position (S1) of signal light byconvergence in the curved surface direction.

Similarly to the above, the focal line position (M21) of stray light 2converged by the anamorphic lens in the curved surface direction isclose to the anamorphic lens than the focal line position (M22) of straylight 2 by convergence in the flat surface direction. The anamorphiclens is designed to make the focal line position (M21) of stray light 2by convergence in the curved surface direction away from the anamorphiclens than the focal line position (S2) of signal light by convergence inthe flat surface direction.

Further, the beam spot of signal light has a shape of a least circle ofconfusion on the convergence position (S0) between the focal lineposition (S1) and the focal line position (S2).

Taking into account the above matters, the following is a descriptionabout a relationship between irradiation areas of signal light and straylight 1, 2 on the plane S0.

As shown in FIG. 2A, the anamorphic lens is divided into four areas Athrough D. In this case, signal light entered into the areas A through Dis distributed on the plane S0, as shown in FIG. 2B. Further, straylight 1 entered into the areas A through D is distributed on the planeS0, as shown in FIG. 2C, and stray light 2 entered into the areas Athrough D is distributed on the plane S0, as shown in FIG. 2D.

If signal light and stray light 1, 2 on the plane S0 are extracted ineach of light flux areas, the distributions of the respective light areas shown in FIGS. 3A through 3D. In this case, stray light 1 and straylight 2 in the same light flux area are not overlapped with signal lightin each of the light flux areas. Accordingly, if the device isconfigured such that only signal light is received by a sensor afterlight fluxes (signal light, stray light 1, 2) in each of the light fluxareas are separated in different directions, only signal light isentered into a corresponding sensor to thereby suppress incidence ofstray light. Thus, it is possible to avoid degradation of a detectionsignal resulting from stray light.

As described above, it is possible to extract only signal light bydispersing and separating light passing through the areas A through Dfrom each other on the plane S0. The embodiment is made based on theabove technical principle.

FIGS. 4A and 4B are diagrams showing a distribution state of signallight and stray light 1, 2 on the plane S0, in the case where thepropagating directions of light fluxes (signal light, stray light 1, 2)passing through the four areas A through D shown in FIG. 2A arerespectively changed in different directions by the same angle. FIG. 4Ais a diagram of the anamorphic lens when viewed from the optical axisdirection of the anamorphic lens (the propagating direction along whichlaser light is entered into the anamorphic lens), and FIG. 4B is adiagram showing a distribution state of signal light, stray light 1, 2on the plane S0.

In FIG. 4A, the propagating directions of light fluxes (signal light,stray light 1, 2) that have passed through the areas A through D arerespectively changed into directions Da, Db, Dc, Dd by the same angleamount α (not shown) with respect to the propagating directions of therespective light fluxes before incidence. The directions Da, Db, Dc, Ddeach has an inclination of 45° with respect to the flat surfacedirection and the curved surface direction.

In this case, as shown in FIG. 4B, it is possible to distribute signallight and stray light 1, 2 in each of the light flux areas, on the planeS0, by adjusting the angle amount a with respect to the directions Da,Db, Dc, Dd. As a result of the above operation, as shown in FIG. 4B, itis possible to form a signal light area where only signal light existson the plane S0. By disposing sensors of a photodetector in the signallight area, it is possible to receive only signal light in each of thelight flux areas by a corresponding sensor.

FIGS. 5A through 5D are diagrams showing a method for arranging sensors.FIG. 5A is a diagram showing light flux areas of reflected light (signallight) on a disc, and FIG. 5B is a diagram showing a distribution stateof signal light on a photodetector, in the case where an anamorphic lensand a photodetector (a four-divided sensor) based on a conventionalastigmatism method are respectively disposed on the arranged position ofthe anamorphic lens and on the plane S0, in the arrangement shown inFIG. 1A. FIGS. 5C and 5D are diagrams showing a distribution state ofsignal light and a sensor layout based on the above principle, on theplane S0.

The direction of a diffraction image (a track image) of signal lightresulting from a track groove has an inclination of 45° with respect tothe flat surface direction and the curved surface direction. In FIG. 5A,assuming that the direction of a track image is aligned with leftwardand rightward directions, in FIGS. 5B through 5D, the direction of atrack image by signal light is aligned in upward and downwarddirections. In FIGS. 5A and 5B, to simplify the description, a lightflux is divided into eight light flux areas a through h. Further, thetrack image is shown by the solid line, and the beam shape in anout-of-focus state is shown by the dotted line.

It is known that an overlapped state of a zero-th order diffractionimage and a first-order diffraction image of signal light resulting froma track groove is obtained by an equation: wavelength/(trackpitch×objective lens NA). As shown in FIGS. 5A, 5B, 5D, a requirementthat a first-order diffraction image is formed in the four light fluxareas a, b, e, h is expressed by: wavelength track pitch×objective lensNA>√2.

In the conventional astigmatism method, sensors P1 through P4 (afour-divided sensor) of a photodetector are arranged as shown in FIG.5B. In this case, assuming that detection signal components based onlight intensities in the light flux areas a through h are expressed by Athrough H, a focus error signal FE and a push-pull signal PP areobtained by the following equations (1) and (2).FE=(A+B+E+F)−(C+D+G+H)  (1)PP=(A+B+G+H)−(C+D+E+F)  (2)

On the other hand, as described above, signal light is distributed inthe signal light area as shown in FIG. 5C in the distribution stateshown in FIG. 4B. In this case, signal light passing through the lightflux areas a through h shown in FIG. 5A is distributed as shown in FIG.5D. Specifically, signal light passing through the light flux areas athrough h in FIG. 5A are guided to the light flux areas a through hshown in FIG. 5D, on the plane S0 where the sensors of the photodetectorare disposed.

Accordingly, by disposing the sensors P11 through P18 at the positionsof the light flux areas a through h shown in FIG. 5D in an overlappedstate as shown in FIG. 5D, it is possible to generate a focus errorsignal and a push-pull signal by performing the same computation asapplied to the process described in the case of FIG. 5B. Specifically,assuming that A through H represent detection signals from the sensorsfor receiving light fluxes in the light flux areas a through h, a focuserror signal FE and a push-pull signal PP can be acquired by the aboveequations (1) and (2) in the same manner as described in the case ofFIG. 5B.

As described above, according to the above principle, it is possible togenerate a focus error signal and a push-pull signal (a tracking errorsignal) with no or less influence of stray light by performing the samecomputation as applied to the process based on the conventionalastigmatism method.

The effect by the above principle is obtained, as shown in FIG. 6, inthe case where the focal line position of stray light 1 in the flatsurface direction is close to the anamorphic lens with respect to theplane S0 (a plane where the beam spot of signal light has a shape of aleast circle of confusion), and the focal line position of stray light 2in the curved surface direction is away from the anamorphic lens withrespect to the plane S0. Specifically, as far as the above relationshipis satisfied, the distribution state of signal light, and stray light 1,2 is as shown in FIG. 4B, which makes it possible to keep signal light,and stray light 1, 2 from overlapping each other on the plane S0. Inother words, as far as the above relationship is satisfied, theadvantage based on the above principle is obtained, even if the focalline position of stray light 1 in the flat surface direction comescloser to the plane S0 than the focal line position of signal light inthe curved surface direction, or even if the focal line position ofstray light 2 in the curved surface direction comes closer to the planeS0 than the focal line position of signal light in the flat surfacedirection.

A spectral element H can be used to distribute signal light passingthrough the eight light flux areas a through h shown in FIG. 5A, on thesensor layout shown in FIG. 5D.

FIG. 7A is a diagram showing an arrangement of the spectral element H.FIG. 7A is a plan view of the spectral element H when viewed from theside of the anamorphic lens shown in FIG. 1B. FIG. 7A also shows theflat surface direction, the curved surface direction of the anamorphiclens shown in FIG. 1B, and a direction of a track image of laser lightto be entered into the spectral element H.

The spectral element H is made of a square transparent plate, and has astepped diffraction pattern (a diffraction hologram) on a light incidentsurface thereof. As shown in FIG. 7A, the light incident surface of thespectral element H is divided into four diffraction areas Ha through Hd.The spectral element H is disposed at such a position that laser lightpassing through the light flux areas A through D shown in FIG. 4A arerespectively entered into the diffraction areas Ha through Hd. Thediffraction areas Ha through Hd respectively diffract the entered laserlight in the directions Da through Dd shown in FIG. 4A by the same angleby diffraction on the diffraction areas Ha through Hd.

FIGS. 7B through 7D are schematic diagrams showing irradiation areas, inthe case where laser light passing through the eight light flux areas athrough h shown in FIG. 5A is irradiated onto the sensor layout shown inFIG. 5D. FIG. 7B is a diagram showing a state as to how signal light isirradiated onto the sensors P11 through P18, in the case where the focusposition of laser light is adjusted on a target recording layer. FIGS.7C, 7D are diagrams showing states of stray light 1 and stray light 2 inthe above condition. To simplify the description, the irradiation areasof laser light passing through the light flux areas a through h areindicated as irradiation areas a through h in each of the drawings ofFIGS. 7B through 7D.

As shown in FIG. 7B, signal light is irradiated onto the sensors P11through P18 based on the above principle. The sensors P11 through P18are configured such that the irradiation area of signal light issufficiently included in each of the sensors P11 through P18.Specifically, as shown in FIG. 7B, the sensor layout is configured insuch a manner that four vertices of the signal light area are positionedon the inside of four vertices on the outside of the sensor layout.

As shown in FIG. 7C, stray light 1 is irradiated onto a positionadjacent to the outside of the signal light area according to the aboveprinciple. As described above, however, if the sensor layout isconfigured in such a manner that the signal light area is positioned onthe inside of the sensor layout, the irradiation area of stray light 1is likely to overlap the sensors P11 through P18. Similarly to theabove, as shown in FIG. 7D, the irradiation area of stray light 2 isalso likely to overlap the sensors P11 through P18.

As described above, in the case where signal light passing through thelight flux areas a through h is distributed on the sensor layout usingthe spectral element H, stray light 1, 2 is likely to be entered intothe sensors P11 through P18, which may degrade the precision of outputsignals from the sensors P11 through P18.

Next, described is an output signal from each sensor resulting frompositional displacement of the sensors P11 through P18, in the casewhere the spectral element H is used.

FIG. 8A is a diagram showing an irradiation area of signal light passingthrough the light flux areas a through h, in the case where thepositions of the sensors P11 through P18 are not displaced. FIG. 8Ashows a state that the focus position of laser light is adjusted on atarget recording layer. As shown in FIG. 8A, in this case, signal lightpassing through the light flux areas a through h is uniformly irradiatedonto each of the sensors.

FIGS. 8B, 8C are enlarged views showing an irradiation area near thesensors P11, P12, and an irradiation area near the sensors P14, P16 inthe state shown in FIG. 8A. As shown in FIGS. 8B, 8C, a slight clearanceis formed between the sensors P11, P12, and between the sensors P14,P16. Likewise, a slight clearance is formed between the sensors P13,P15, and between the sensors P17, P18.

As shown in FIG. 8B, although an upper end of the irradiation area a anda lower end of the irradiation area h are respectively deviated from thesensors P11, P12, the irradiation areas a, h respectively and uniformlyoverlap the sensors P11, P12. As shown in FIG. 8C, although a left endof the irradiation area b and a right end of the irradiation area c arerespectively deviated from the sensors P16, P14, the irradiation areasb, c respectively and uniformly overlap the sensors P16, P14. Likewise,the irradiation areas f, g respectively and uniformly overlap thesensors P13, P15, and the irradiation areas d, e respectively anduniformly overlap the sensors P17, P18.

FIG. 8D is a diagram showing irradiation areas of signal light passingthrough the light flux areas a through h, in the case where thepositions of the sensors P11 through P18 are displaced from the stateshown in FIG. 8A in a direction (leftward or rightward direction)perpendicular to the direction of a track image. As shown in FIG. 8D,although the irradiation areas are the same as those in the state shownin FIG. 8A, since the positions of the sensors P11 through P18 aredisplaced leftward, the irradiation areas in the state shown in FIG. 8Dare displaced rightward within the sensors P11 through P18.

FIG. 8E is an enlarged view showing irradiation areas near the sensorsP11, P12 in the state shown in FIG. 8D. As shown in FIG. 8E, theirradiation areas a, h respectively and uniformly overlap the sensorsP11, P12 in the same manner as the state shown in FIG. 8B, although theirradiation areas a, h are respectively displaced rightward from thesensors P11, P12. Accordingly, the output signals from the sensors P11,P12 in the state shown in FIG. 8E are substantially the same as theoutput signals from the sensors P11, P12 in the state shown in FIG. 8A.Likewise, the output signals from the sensors P17, P18 in the stateshown in FIG. 8E are substantially the same as the output signals fromthe sensors P17, P18 in the state shown in FIG. 8A.

FIG. 8F is an enlarged view showing irradiation areas near the sensorsP14, P16 in the state shown in FIG. 8D. As shown in FIG. 8F, although aright end of the irradiation area b lies within the sensor P16, a leftend of the irradiation area b overlaps the sensor P16, unlike the stateshown in FIG. 8C. Further, although a left end of the irradiation area clies within the sensor P14, a right end of the irradiation area c isdeviated rightward from the sensor P14 and overlaps the sensor P16,unlike the state shown in FIG. 8C. As a result, the output signal fromthe sensor P16 is increased, and the output signal from the sensor P14is decreased, as compared with the state shown in FIG. 8A. Likewise, theoutput signal from the sensor P15 is increased, and the output signalfrom the sensor P13 is decreased, as compared with the state shown inFIG. 8A.

Further, in the case where the positions of the sensors P11 through P18are displaced rightward substantially by the same displacement amount asthe state shown in FIG. 8D, the output signals from the sensors P11,P12, P17, P18 are kept substantially unchanged, the output signals fromthe sensors P13, P14 are increased, and the output signals from thesensors P15, P16 are decreased, as compared with the state shown in FIG.8A. Further, in the case where the positions of the sensors P11 throughP18 are displaced in a direction (upward or downward direction) inparallel to the direction of a track image substantially by the samedisplacement amount as the state shown in FIG. 8D, the output signalsfrom the sensors P13 through P16 are kept substantially unchanged, andthe output signals from the sensors P11, P12, P17, P18 are changed.

In the above arrangement, it is preferable to keep the output signalsfrom the sensors P11 through P18 unchanged, even if the positions of thesensors P11 through P18 are displaced. However, as described above, ifthe positions of the sensors P11 through P18 are displaced, the outputsignals from the sensors P11 through P18 are changed depending on adirection of the positional displacement and an amount of the positionaldisplacement. As a result, the precision of output signals from thesensors P11 through P18 may be lowered.

Next, there is described an approach of suppressing an offset (DCcomponent) of a push-pull signal resulting from positional displacementof the sensors P11 through P18, in the case where the spectral element His used. An example of the approach is disclosed in Japanese PatentApplication No. 2009-10369 (corresponding to U.S. Patent ApplicationPublication No. US2010/0182891 A1) previously filed by the applicant ofthe present application. The disclosures of the above applications areincorporated herein by reference.

FIG. 9 is a diagram showing a circuit configuration for suppressing anoffset (a DC component) of a push-pull signal. The push-pull signalgeneration circuit in the above case is provided with adder circuits 11,12, 14, 15, subtractor circuits 13, 18, 20, gain circuits 16, 17, amultiplier circuit 19, and a comparator/computer 21.

The adder circuit 11 sums up output signals from the sensors P11, P12,and outputs a signal PP1L in accordance with the light amount ofleft-side signal light. The adder circuit 12 sums up output signals fromthe sensors P17, P18, and outputs a signal PP1R in accordance with thelight amount of right-side signal light. The subtractor circuit 13computes a difference between output signals from the adder circuits 11,12, and generates a signal PP1 based on a light amount differencebetween the left and right two signal light.

The adder circuit 14 sums up output signals from the sensors P13, P14,and outputs a signal PP2L in accordance with the light amount ofleft-side signal light of upper and lower two signal light. The addercircuit 15 sums up output signals from the sensors P15, P16, and outputsa signal PP2R in accordance with the light amount of right-side signallight of upper and lower two signal light. The gain circuits 16, 17multiplies the outputs from the adder circuits 14, 15 by gains α, βunder the control of the comparator/computer 21. The subtractor circuit18 computes a difference between output signals from the gain circuits16, 17, and generates a signal PP2 based on alight amount difference inleftward or rightward direction between the upper and lower two signallight.

The multiplier circuit 19 outputs a signal obtained by multiplying thesignal PP2 to be outputted from the subtractor circuit 18 with avariable k to the subtractor circuit 20. In this arrangement, thevariable k is set to a value that suppresses an offset (a DC component)of a push-pull signal resulting from lens shift. A concrete settingmethod of the variable k is disclosed in Japanese Unexamined PatentPublication No. 2010-102813 (corresponding to U.S. Patent ApplicationPublication No. US2010/0080106 A1) of the patent application previouslyfiled by the applicant of the present application. The disclosures ofthe above applications are incorporated herein by reference.

The subtractor circuit 20 subtracts a signal to be inputted from themultiplier circuit 19, from the signal PP1 to be inputted from thesubtractor circuit 13; and outputs a signal after the subtraction as apush-pull signal.

The comparator/computer 21 adjusts the gains α, β of the gain circuits16 17 based on signals from the adder circuits 14, 15. Thecomparator/computer 21 calculates the gains α, β by the followingequations (3), (4) in a state that the optical axes of the objectivelens and laser light are not displaced from each other (no lens shift)immediately after laser light is focused on a disc (a state thattracking servo control is turned off).α={(PP2L+PP2R)/2}/PP2L  (3)β={(PP2L+PP2R)/2}/PP2R  (4)

The comparator/computer 21 calculates the gains α, β by the equations(3), (4), and sets the calculated gains α, β in the gain circuits 16,17.

As described above, by setting the gains α, β, it is possible to correctimbalance of signals from the adder circuits 14, 15, even in the casewhere the positions of the sensors P11 through P18 are displaced in adirection (leftward or rightward direction) perpendicular to thedirection of a track image. With this arrangement, it is possible tosuppress an offset (a DC component) of a push-pull signal based on adisplacement amount, in the case where the positions of the sensors P11through P18 are displaced in leftward or rightward direction.

In the above arrangement, if the positions of the sensors P11 throughP18 are displaced in leftward or rightward direction, as described abovereferring to FIGS. 8D through 8F, output signals from the sensors P13through P16 are changed depending on a displacement amount. Further,output signals from the adder circuits 14, 15 are changed depending on achange in the output signals from the sensors P13 through P16. In viewof the above, it is necessary to properly set the gains α, β dependingon a displacement amount of the sensors P11 through P18 in leftward orrightward direction. In this case, if positional displacement of thesensors P11 through P18 is increased, the gains α, β are also increasedin accordance with an increase in the positional displacement. Anincrease in the gains α, β results in an increase of noise which may besuperimposed on the signals PP2L, PP2R, which may resultantly lower theprecision of a push-pull signal.

In the following description on an example, there is described animprovement on the spectral element H that enables to suppress theaforementioned drawbacks, as well as a concrete construction example ofthe optical pickup device.

EXAMPLE

The inventive example is an example, wherein the invention is applied toan optical pickup device compatible with BD, DVD and CD. The aboveprinciple is applied only to an optical system for BD, and a focusadjusting technology by a conventional astigmatism method and a trackingadjusting technology by a 3-beam system (an in-line system) are appliedto an optical system for CD and an optical system for DVD.

FIGS. 10A and 10B are diagrams showing an optical system of an opticalpickup device in the inventive example. FIG. 10A is a plan view of theoptical system showing a state that elements of the optical system onthe disc side with respect to rise-up mirrors 114, 115 are omitted, andFIG. 10B is a perspective side view of the optical system posterior tothe rise-up mirrors 114, 115.

As shown in FIG. 10A, the optical pickup device is provided with asemiconductor laser 101, a half wave plate 102, a diverging lens 103, adual wavelength laser 104, a diffraction grating 105, a diverging lens106, a complex prism 107, a front monitor 108, a collimator lens 109, adriving mechanism 110, reflection mirrors 111, 112, a quarter wave plate113, the rise-up mirrors 114, 115, a dual wavelength objective lens 116,a BD objective lens 117, a spectral element 118, an anamorphic lens 119,and a photodetector 120.

The semiconductor laser 101 emits laser light (hereinafter, called as“BD light”) for BD and having a wavelength of or about 405 nm. The halfwave plate 102 adjusts the polarization direction of BD light. Thediverging lens 103 adjusts the focal length of BD light to shorten thedistance between the semiconductor laser 101 and the complex prism 107.

The dual wavelength laser 104 accommodates, in a certain CAN, two laserelements which each emit laser light (hereinafter, called as “CD light”)for CD and having a wavelength of or about 785 nm, and laser light(hereinafter, called as “DVD light”) for DVD and having a wavelength ofor about 660 nm.

FIG. 10C is a diagram showing an arrangement pattern of laser elements(laser light sources) in the dual wavelength laser 104. FIG. 10C is adiagram of the dual wavelength laser 104 when viewed from the beamemission side. In FIG. 10C, CE and DE respectively indicate emissionpoints of CD light and DVD light. The gap between the emission points ofCD light and DVD light is represented by the symbol G.

As will be described later, the gap G between the emission point CE ofCD light and the emission point DE of DVD light is set to such a valueas to properly irradiate DVD light onto a four-divided sensor for DVDlight. Accommodating two light sources in one CAN as described aboveenables to simplify the optical system, as compared with an arrangementprovided with plural CANs.

Referring back to FIG. 10A, the diffraction grating 105 separates eachof CD light and DVD light into a main beam and two sub beams. Thediffraction grating 105 is a two-step diffraction grating. Further, thediffraction grating 105 is integrally formed with a half wave plate. Thehalf wave plate integrally formed with the diffraction grating 105adjusts the polarization directions of CD light and DVD light. Thediverging lens 106 adjusts the focal lengths of CD light and DVD lightto shorten the distance between the dual wavelength laser 104 and thecomplex prism 107.

The complex prism 107 is internally formed with a dichroic surface 107a, and a Polarizing Beam Splitter (PBS) surface 107 b. The dichroicsurface 107 a reflects BD light, and transmits CD light and DVD light.The semiconductor laser 101, the dual wavelength laser 104 and thecomplex prism 107 are disposed at such positions that the optical axisof BD light reflected on the dichroic surface 107 a and the optical axisof CD light transmitted through the dichroic surface 107 a are alignedwith each other. The optical axis of DVD light transmitted through thedichroic surface 107 a is displaced from the optical axes of BD lightand CD light by the gap G shown in FIG. 10C.

A part of each of BD light, CD light and DVD light is reflected on thePBS surface 107 b, and a main part thereof is transmitted through thePBS surface 107 b. As described above, the half wave plate 102, and thediffraction grating 105 (and the half wave plate integrally formed withthe diffraction grating 105) are disposed at such positions that a partof each of BD light, CD light and DVD light is reflected on the PBSsurface 107 b.

When the diffraction grating 105 is disposed at the position asdescribed above, a main beam and two sub beams of CD light, and a mainbeam and two sub beams of DVD light are respectively aligned along thetracks of CD and DVD. The main beam and the two sub beams reflected onCD are irradiated onto four-divided sensors for CD on the photodetector120, which will be described later. The main beam and two sub beamsreflected on DVD are irradiated onto four-divided sensors for DVD on thephotodetector 120, which will be described later.

BD light, CD light, DVD light reflected on the PBS surface 107 b isirradiated onto the front monitor 108. The front monitor 108 outputs asignal in accordance with a received light amount. The signal from thefront monitor 108 is used for emission power control of thesemiconductor laser 101 and the dual wavelength laser 104.

The collimator lens 109 converts BD light, CD light and DVD lightentered from the side of the complex prism 107 into parallel light. Thedriving mechanism 110 moves the collimator lens 109 in the optical axisdirection in accordance with a control signal for aberration correction.The driving mechanism 110 is provided with a holder 110 a for holdingthe collimator lens 109, and a gear 110 b for feeding the holder 110 ain the optical axis direction of the collimator lens 109. The gear 110 bis interconnected to a driving shaft of a motor 110 c.

BD light, CD light and DVD light collimated by the collimator lens 109are reflected on the two reflection mirrors 111, 112, and are enteredinto the quarter wave plate 113. The quarter wave plate 113 converts BDlight, CD light and DVD light entered from the side of the reflectionmirror 112 into circularly polarized light, and converts BD light, CDlight and DVD light entered from the side of the rise-up mirror 114 intoa linearly polarized light whose polarization direction is orthogonal tothe polarization direction upon incidence from the side of thereflection mirror 112. With this operation, light reflected on a disc isreflected on the PBS surface 107 b.

The rise-up mirror 114 is a dichroic mirror. The rise-up mirror 114transmits BD light, and reflects CD light and DVD light in a directiontoward the dual wavelength objective lens 116. The rise-up mirror 115reflects BD light in a direction toward the BD objective lens 117.

The dual wavelength objective lens 116 is configured to properly focusCD light and DVD light on CD and DVD, respectively. Further, the BDobjective lens 117 is configured to properly focus BD light on BD. Thedual wavelength objective lens 116 and the BD objective lens 117 aredriven by an objective lens actuator 132 in a focus direction and in atracking direction, while being held on the holder 110 a.

The spectral element 118 has a stepped diffraction pattern (adiffraction hologram) on an incident surface thereof. Out of BD light,CD light and DVD light entered into the spectral element 118, BD lightis divided into twelve light fluxes, which will be described later, andthe propagating direction of each of the light fluxes is changed bydiffraction on the spectral element 118. Main parts of CD light and DVDlight are transmitted through the spectral element 118 withoutdiffraction on the spectral element 118. An arrangement of the spectralelement 118 will be described later referring to FIG. 11A.

The anamorphic lens 119 imparts astigmatism to BD light, CD light andDVD light entered from the side of the spectral element 118. Theanamorphic lens 119 corresponds to the anamorphic lens shown in FIG. 1B.BD light, CD light and DVD light transmitted through the anamorphic lens119 are entered into the photodetector 120. The photodetector 120 has asensor layout for receiving the respective light. The sensor layout ofthe photodetector 120 will be described later referring to FIG. 12.

FIG. 11A is a diagram showing an arrangement of the spectral element118. FIG. 11A is a plan view of the spectral element 118, when viewedfrom the side of the complex prism 107. FIG. 11A also shows the flatsurface direction, the curved surface direction of the anamorphic lens119, and a direction of a track image of laser light to be entered intothe spectral element 118.

The spectral element 118 is made of a square transparent plate, and hasa stepped diffraction pattern (a diffraction hologram) on a lightincident surface thereof. The diffraction pattern is a steppeddiffraction pattern. The step number and the step height of thediffraction pattern are set such that plus first order diffractionefficiency with respect to the wavelength of BD light is set high, andthat zero-th order diffraction efficiency with respect to thewavelengths of CD light and DVD light is set high.

As shown in FIG. 11A, the light incident surface of the spectral element118 is divided into sixteen diffraction areas 118 a 0 through 118 h 0,118 a 1 through 118 h 1. The spectral element 118 is disposed at such aposition that BD light is uniformly entered into each of the diffractionareas 118 a 0 through 118 h 0, 118 a 1 through 118 h 1. Specifically,the spectral element 118 is disposed at such a position that the centerof the spectral element 118 shown in FIG. 11A is aligned with theoptical axis of BD light.

As shown in FIG. 11A, an area (hereinafter, called as a “verticallyoriented area”) obtained by combining the diffraction areas 118 a 1, 118d 1, 118 e 1, 118 h 1 extends in a direction perpendicular to thedirection of a track image of laser light, and has a width d. An area(hereinafter, called as a “transversely oriented area”) obtained bycombining the diffraction areas 118 b 1, 118 c 1, 118 f 1, 118 g 1extends in a direction in parallel to the direction of a track image oflaser light, and has a width d.

FIG. 11B is a diagram showing light flux areas a0 through h0, a1 throughh1 of BD light which is entered into the diffraction areas 118 a 0through 118 h 0, 118 a 1 through 118 h 1 of the spectral element 118.Light passing through light flux areas a0 through h0, a1 through h1 isrespectively entered into the diffraction areas 118 a 0 through 118 h 0,118 a 1 through 118 h 1.

Referring back to FIG. 11A, the diffraction areas 118 a 0 through 118 h0 diffract entered BD light in directions Va0 through Vh0 by plus firstorder diffraction. The directions Va0, Vh0, the directions Vf0, Vg0, thedirections Vb0, Vc0, the directions Vd0, Ve0 respectively coincide withthe directions Da through Dd shown in FIG. 4A. Further, each of thediffraction areas 118 a 0 through 118 h 0 diffracts BD light by the samediffraction angle by plus first order diffraction. The diffraction angleis adjusted by the pitch of a diffraction pattern.

The diffraction areas 118 a 1 through 118 h 1 diffract entered BD lightin directions Va1 through Vh1 by plus first order diffraction. As shownin FIG. 11A, the directions Va1 through Vh1 are inclined by 45° withrespect to the directions Va0 through Vh0. Further, as shown in FIG.11A, the directions Va1, Vd1, the directions Ve1, Vh1 are respectivelydirections in parallel to the flat surface direction and different fromeach other; and the directions Vb1, Vg1, the directions Vc1, Vf1 arerespectively directions in parallel to the curved surface direction anddifferent from each other. The pitch of the diffraction pattern on thediffraction areas 118 a 1 through 118 h 1 is set smaller than the pitchof the diffraction pattern on the diffraction areas 118 a 0 through 118h 0. With this arrangement, the diffraction angle of BD light diffractedon the diffraction areas 118 a 1 through 118 h 1 is set larger than thediffraction angle of BD light diffracted on the diffraction areas 118 a0 through 118 h 0.

With use of the spectral element 118 having the above configuration, BDlight diffracted on the diffraction areas 118 a 0 through 118 h 0 isirradiated onto the light receiving surface of the photodetector 120, asshown in FIG. 4B. Further, as will be described later, BD lightdiffracted on the diffraction areas 118 a 1 through 118 h 1 isirradiated onto a position on the outside of a rectangle defined byvertices on the outside of the sensor layout, on the light receivingsurface of the photodetector 120. CD light and DVD light are irradiatedonto four-divided sensors on the photodetector 120, which will bedescribed later, substantially without diffraction on the diffractionareas 118 a 0 through 118 h 0, 118 a 1 through 118 h 1.

The diffraction areas 118 a 0 through 118 h 0, 118 a 1 through 118 h 1are formed by e.g. a diffraction pattern having eight steps. In thiscase, the step difference per step is set to 7.35 μm. With thisarrangement, it is possible to set the diffraction efficiencies ofzero-th order diffraction light of CD light and DVD light to 99% and 92%respectively, while keeping the diffraction efficiency of plus firstorder diffraction light of BD light to 81%. In this case, zero-th orderdiffraction efficiency of BD light is set to 7%.

Alternatively, it is possible to set the number of steps of adiffraction pattern to be formed in the diffraction areas 118 a 0through 118 h 0, 118 a 1 through 118 h 1 to the number other than eight.Furthermore, it is possible to configure the diffraction areas 118 a 0through 118 h 0, 118 a 1 through 118 h 1 by using e.g. the technologydisclosed in Japanese Unexamined Patent Publication No. 2006-73042.Using the above technology enables to more finely adjust diffractionefficiencies of BD light, CD light, and DVD light.

FIG. 12 is a diagram showing a sensor layout of the photodetector 120.

The photodetector 120 has sensors B1 through B8 for BD and for receivingBD light separated by the spectral element 118; four-divided sensors C01through C03 for CD and for receiving CD light transmitted through thespectral element 118 without separation by the spectral element 118; andfour-divided sensors D01 through D03 for DVD and for receiving DVD lighttransmitted through the spectral element 118 without separation by thespectral element 118. Signal light of BD light separated by the spectralelement 118 is respectively irradiated onto vertex portions of thesignal light area, in the same manner as the irradiation area of signallight shown in FIG. 4B.

As shown in FIG. 12, the sensors B1, B2, the sensors B3, B5, the sensorsB4, B6, the sensors B7, B8 are respectively disposed near the fourvertices of the signal light area to receive signal light of BD lightpassing through the light flux areas a0 through h0, respectively. Thesensors B1 through B8 are disposed at such positions that theirradiation area of BD light which is positioned on the inside of thefour vertex portions of the signal light area is sufficiently included.With this arrangement, it is possible to sufficiently receive signallight separated by the spectral element 118 by the sensors B1 throughB8, even in the case where the positions of the sensors B1 through B8are displaced resulting from e.g. aging deterioration.

The optical axes of BD light and CD light are aligned with each other onthe dichroic surface 107 a as described above. Accordingly, a main beam(zero-th order diffraction light) of CD light is irradiated onto acenter of the signal light area of BD light, on the light receivingsurface of the photodetector 120. The four-divided sensor C01 isdisposed at the center position of a main beam of CD light. Thefour-divided sensors C02, C03 are disposed in the direction of a trackimage with respect to a main beam of CD light, on the light receivingsurface of the photodetector 120, to receive sub beams of CD light.

Since the optical axis of DVD light is displaced from the optical axisof CD light as described above, a main beam and two sub beams of DVDlight are irradiated at positions displaced from the irradiationpositions of a main beam and two sub beams of CD light, on the lightreceiving surface of the photodetector 120. The four-divided sensors D01through D03 are respectively disposed at the irradiation positions of amain beam and two sub beams of DVD light. The distance between a mainbeam of CD light and a main beam of DVD light is determined by the gap Gbetween emission points of CD light and DVD light shown in FIG. 10C.

FIGS. 13A through 13C are schematic diagrams showing irradiation areasof BD light, in the case where BD light passing through the sixteenlight flux areas a0 through h0, a1 through h1 shown in FIG. 11B isirradiated onto the sensors B1 through B8 shown in FIG. 12. FIGS. 13Athrough 13C are diagrams respectively showing signal light, stray light1 and stray light 2 of BD light that is irradiated onto the sensors B1through B8, in the case where the focus position of BD light is adjustedon a target recording layer. To simplify the description, theirradiation areas of BD light passing through the light flux areas a0through h0, a1 through h1 are indicated as irradiation areas a0 throughh0, a1 through h1 in each of the drawings of FIGS. 13A through 13C.Further, to simplify the description, the shape of the sensors B1through B8 shown in FIGS. 13A through 13C is simplified in comparisonwith the shape of the sensors B1 through B8 shown in FIG. 12.

As shown in FIG. 13A, signal light of BD light passing through the lightflux areas a0 through h0 is irradiated onto the sensors B1 through B8,and signal light of BD light passing through the light flux areas a1through h1 is irradiated onto a position away from the signal lightarea. Out of signal light of BD light to be entered into the spectralelement 118, signal light of BD light to be entered into the diffractionareas 118 a 1 through 118 h 1 is diffracted with a large diffractionangle toward the outside of the signal light area. As a result of theabove operation, out of signal light of BD light to be entered into thespectral element 118, only signal light of BD light to be entered intothe diffraction areas 118 a 0 through 118 h 0 is irradiated onto thesensors B1 through B8. In this arrangement, the irradiation areas a0,h0, the irradiation areas b0, c0, the irradiation areas f0, g0, and theirradiation areas d0, e0 each has a shape obtained by removing a centerportion from a corresponding one of the four irradiation areas (see FIG.7B) obtained in the case where the spectral element H is used, dependingon the width d shown in FIG. 11A.

As shown in FIGS. 13B, 13C, stray light 1, 2 of BD light passing throughthe light flux areas a0 through h0, a1 through h1 are irradiated onto aposition on the outside of the signal light area. In this case, there isformed a clearance between the irradiation areas a0, h0, and likewise,there is formed a clearance between the irradiation areas b0, c0,between the irradiation areas f0, g0, the irradiation areas d0, e0. Theclearance is formed depending on the width d shown in FIG. 11A.Specifically, the irradiation areas a0, h0, the irradiation areas b0,c0, the irradiation areas f0, g0, the irradiation areas d0, e0 each hasa shape obtained by removing a boundary portion between the respectivetwo irradiation areas from a corresponding one of the irradiation areasin the state shown in FIGS. 7C, 7D obtained in the case where thespectral element H is used, by the diffraction areas 118 a 1 through 118h 1 shown in FIG. 11A. As shown in FIGS. 13B, 13C, these clearancesextend near the vertices of the signal light area. With thisarrangement, there is no or less likelihood that stray light 1, 2 may beentered into the sensors near the vertices of the signal light area.

Next, described is an output signal from each sensor, in the case wherethe positions of the sensors P11 through P18 are displaced in theinventive example.

FIG. 14A is a diagram showing an irradiation area of signal lightpassing through the light flux areas a0 through h0, in the case wherethe positions of the sensors B1 through B8 are not displaced. FIG. 14Ashows a state that the focus position of laser light is adjusted on atarget recording layer. In this case, as shown in FIG. 14A, signal lightpassing through the light flux areas a0 through h0 is uniformlyirradiated onto each of the sensors. There is formed a slight clearancebetween the sensors B1, B2, between the sensors B4, B6, between thesensors B3, B5, between the sensors B7, B8.

FIGS. 14B, 14C are enlarged views showing an irradiation area near thesensors B1, B2, and an irradiation area near the sensors B4, B6,respectively, in the case shown in FIG. 14A. The hatched portions inbetween the irradiation areas in FIGS. 14B, 14C respectively indicateareas, with signal light being removed by the diffraction areas 118 a 1,118 h 1, the diffraction areas 118 b 1, 118 c 1 of the spectral element118. Specifically, in the case where the spectral element H is used inplace of the spectral element 118, BD light passing through the lightflux areas a, h, and the light flux areas b, c is irradiated onto areasobtained by adding the hatched portions to the broken-line portions.

As shown in FIG. 14B, the irradiation area a0 has a shape with an upperend of the irradiation area a shown in FIG. 8B being removed, and theirradiation area h0 has a shape with a lower end of the irradiation areah shown in FIG. 8B being removed. The irradiation areas a0, h0respectively and uniformly overlap the sensors B1, B2. Further, as shownin FIG. 14C, the irradiation area b0 has a shape with a left end of theirradiation area b shown in FIG. 8C being removed, and the irradiationarea c0 has a shape with a right end of the irradiation area c shown inFIG. 8C being removed. The irradiation areas b0, c0 respectively anduniformly overlap the sensors B6, B4. Likewise, the irradiation areasf0, g0 respectively and uniformly overlap the sensors B3, B5, and theirradiation areas d0, e0 respectively and uniformly overlap the sensorsB7, B8.

FIG. 14D is a diagram showing an irradiation area of signal lightpassing through the light flux areas a0 through h0, in the case wherethe positions of the sensors B1 through B8 are displaced from the stateshown in FIG. 14A in a direction (leftward or rightward direction)perpendicular to the direction of a track image. In this case, as shownin FIG. 14D, although the irradiation areas are the same as those in thestate shown in FIG. 14A, since the positions of the sensors B1 throughB8 are displaced leftward, the irradiation areas in FIG. 14D aredisplaced rightward within the sensors B1 through B8.

FIG. 14E is an enlarged view showing irradiation areas near the sensorsB1, B2 in the state shown in FIG. 14D. As shown in FIG. 14E, theirradiation areas a0, h0 respectively and uniformly overlap the sensorsB1, B2 in the same manner as the state shown in FIG. 14B, although theirradiation areas a0, h0 are respectively shifted rightward with respectto the sensors B1, B2. Accordingly, the output signals from the sensorsB1, B2 in FIG. 14E are substantially the same as the output signals fromthe sensors B1, B2 in the state shown in FIG. 14A. Likewise, the outputsignals from the sensors B7, B8 in FIG. 14E are substantially the sameas the output signals from the sensors B7, B8 in the state shown in FIG.14A.

FIG. 14F is an enlarged view showing irradiation areas near the sensorsB4, B6 in the state shown in FIG. 14D. As shown in FIG. 14F, theirradiation area b0 lies within the sensor B6 in the same manner as thestate shown in FIG. 14C. Further, the irradiation area c0 lies withinthe sensor B4 in the same manner as the state shown in FIG. 14C.Accordingly, the output signals from the sensors B4, B6 in FIG. 14F arerespectively substantially the same as the output signals from thesensors B4, B6 in the state shown in FIG. 14A. Likewise, the outputsignal from the sensors B3, B5 in FIG. 14F are respectivelysubstantially the same as the output signals from the sensors B3, B5 inthe state shown in FIG. 14A.

Further, in the case where the positions of the sensors B1 through B8are displaced rightward substantially by the same displacement amount asthe state shown in FIG. 14D, the output signals from the sensors B1through B8 are kept substantially unchanged in the same manner as thestates shown in FIGS. 14D through 14F. Accordingly, even if thepositions of the sensors B1 through B8 are displaced in a direction(upward or downward direction) in parallel to the direction of a trackimage substantially by the same displacement amount as the state shownin FIG. 14D, the output signals from the sensors B1 through B8 are keptsubstantially unchanged.

In the above arrangement, it is preferable to set the clearance betweenthe respective two irradiation areas positioned at four vertex portionsof the signal light area larger than the clearance between therespective two sensors corresponding to the respective two irradiationareas so as to keep the output signals from the sensors B1 through B8substantially unchanged, even if the positions of the sensors B1 throughB8 are displaced. The clearance between the respective two irradiationareas is properly adjusted by the width d shown in FIG. 11A.

Next, there is described an arrangement of the inventive example in thecase where the approach of suppressing an offset (a DC component) of apush-pull signal resulting from positional displacement of a sensor, asdescribed above referring to FIG. 9.

In the inventive example, as shown in FIGS. 14D through 14F, even in thecase where the positions of the sensors B1 through B8 are displaced in adirection in parallel to the direction of a track image, and in adirection perpendicular to the direction of a track image, the outputsignals from the sensors B1 through B8 are kept substantially unchanged.Accordingly, as shown in FIGS. 14D through 14F, even in the case wherethe positions of the sensors B1 through B8 are displaced in a direction(leftward or rightward direction) perpendicular to the direction of atrack image, unlike the case where the spectral element H is used,signals from the adder circuits 14, 15 shown in FIG. 9 are balanced. Inthis case, it is not specifically necessary to set the gains α, β inorder to adjust the signal imbalance.

Next, FIGS. 15A through 15D show simulation results of irradiation areason the sensor layout, in the case where the spectral element H is used,and in the case where the spectral element 118 in the inventive exampleis used.

FIG. 15A shows a distribution state of signal light, stray light 1, 2 onthe light receiving surface, in the case where the spectral element Hshown in FIG. 7A is used, and FIG. 15B is an enlarged view of a rightvertex portion in the state shown in FIG. 15A. FIG. 15C shows adistribution state of signal light, stray light 1, 2 on the lightreceiving surface, in the case where the spectral element 118 is used,and FIG. 15D is an enlarged view of a right vertex portion in the stateshown in FIG. 15C. FIGS. 15A through 15D respectively show simulationresults in the case where there is no lens shift.

FIG. 16A shows a distribution state of signal light, stray light 1, 2 onthe light receiving surface, in the case where the spectral element H isused, and FIG. 16B is an enlarged view of a right vertex portion in thestate shown in FIG. 16A. FIG. 16C shows a distribution state of signallight, stray light 1, 2 on the light receiving surface, in the casewhere the spectral element 118 is used, and FIG. 16D is an enlarged viewof a right vertex portion in the state shown in FIG. 16C. FIGS. 16Athrough 16D respectively show simulation results in the case where alens is shifted by 300 μm.

In the above simulation, the width d of the spectral element 118 is setto 10% of the diameter of laser light to be entered into the spectralelement 118. Further, the above simulation is made based on the premisethat the BD objective lens 117 is not shifted in FIGS. 15A through 15D,and that the BD objective lens 117 is shifted by 300 μm in FIGS. 16Athrough 16D. Furthermore, in the above simulation, a light receivingsensor is constituted of the sensors B1 through B8 in the inventiveexample.

As shown in FIGS. 15A and 15B, FIGS. 16A and 16B, in the case where thespectral element H is used, the irradiation area of stray light comesclose to the irradiation area of signal light. As a result, stray lightis likely to be entered into the sensors B1 through B8. In contrast, asshown in FIGS. 15C and 15D, FIGS. 16C and 16D, in the case where thespectral element 118 in the inventive example is used, there is no orless likelihood that stray light may be entered into the sensors B1through B8, because irradiation areas of stray light which entered tothe positions near the vertices of the signal light area, are removed bydiffraction on the diffraction areas 118 a 1 through 118 h 1 shown inFIG. 11A, as compared with the case where the spectral element H isused.

As described above, in the inventive example, there is no or lesslikelihood that stray light 1, 2 may be superimposed on signal light ofBD light, as compared with the case where the spectral element H isused. Thus, the inventive example is advantageous in enhancing theprecision of output signals from the sensors B1 through B8 based onsignal light of BD light.

Further, in the inventive example, as shown in FIGS. 14D through 14F,even if the positions of the sensors B1 through B8 are displaced, theoutput signal from each sensor is kept substantially unchanged, ascompared with the case where the spectral element H is used. Thus, theinventive example is advantageous in enhancing the precision of outputsignals from the sensors B1 through B8, even if the positions of thesensors B1 through B8 are displaced.

Furthermore, in the inventive example, in the case where the approach(see FIG. 9) of suppressing an offset (a DC component) of a push-pullsignal resulting from positional displacement of a sensor is used, asshown in FIGS. 14D through 14F, even if the positions of the sensors B1through B8 are displaced, signals from the adder circuit 14, 15 shown inFIG. 9 are balanced, unlike the case where the spectral element H isused. Thus, there is no or less necessity that the gains α, β be seteach time the positions of the sensors P11 through P18 are displaced inleftward or rightward direction resulting from e.g. aging deterioration,once the gains α, β are set.

The inventive example is advantageous even in the case where apositional displacement amount of the sensors B1 through B8 is largerthan the positional displacement amount shown in FIG. 14D, and thesignal light area to be formed by signal light of BD light is deviatedfrom a rectangle defined by the vertices on the outside of the sensorlayout. Specifically, in the inventive example, even in the case wherepositional displacement of the sensors B1 through B8 is large, theamount by which each of the irradiation areas is deviated from acorresponding sensor, and the amount by which each of the irradiationareas overlaps a sensor adjacent to the corresponding sensor aredecreased, as compared with the case where the spectral element H isused. Thus, it is possible to enhance the precision of output signalsfrom the sensors B1 through B8 in the above case, as compared with thecase where the spectral element H is used. Further, since imbalance ofsignals from the adder circuits 14, 15 shown in FIG. 9 is reduced in theabove case, it is possible to set the gains α, β to a small value. Asdescribed above, in the inventive example, since the gains α, β can beset to a small value, as compared with the case where the spectralelement H shown in FIG. 7A is used, noise is not excessively amplified,even if the noise is superimposed on the signal PP2L, PP2R. Thus, it ispossible to obtain a satisfactory push-pull signal PP.

The example of the invention has been described as above. The inventionis not limited to the foregoing example, and the example of theinvention may be modified in various ways other than the above.

For instance, in the inventive example, as shown in FIG. 11A, BD lightis diffracted on diffraction areas adjacent to each other, out of thediffraction areas 118 a 1 through 118 h 1, in the flat surface directionor the curved surface direction, and in directions different form eachother. Alternatively, the diffraction direction may be set, asnecessary, in such a manner that diffracted BD light is not irradiatedonto the sensors B1 through B8. Further alternatively, diffraction areasadjacent to each other, out of the diffraction areas 118 a 1 through 118h 1, may be integrally formed into one diffraction area. In the abovecase, the diffraction direction may also be set, as necessary, in such amanner that diffracted BD light is not irradiated onto the sensors B1through B8.

Furthermore, in the inventive example, the vertically oriented area andthe transversely oriented area having the width d are formed alongstraight lines inclined from the flat surface direction and from thecurved surface direction by 45°. Alternatively, the vertically orientedarea and the transversely oriented area may be formed with a lightblocking portion where incidence of laser light is blocked. In the abovecase, signal light of BD light is irradiated onto the sensors B1 throughB8 in the same manner as the inventive example. In this case, the lightamount of CD light to be irradiated onto the four-divided sensors C01through C03, and the light amount of DVD light to be irradiated onto thefour-divided sensors D01 through D03 are reduced by the light blockingportions. In the case where the reduction in the light amount of CDlight and DVD light causes a problem, the optical system for receivingBD light, and the optical system for receiving CD light and DVD lightmay be individually constructed.

Further, in the inventive example, BD light is separated by the spectralelement 118 having a diffraction pattern on a light incident surfacethereof. Alternatively, BD light may be separated by using a spectralelement constituted of a multifaceted prism.

In the case where a spectral element constituted of a multifaceted prismis used, the optical system for receiving BD light, and the opticalsystem for receiving CD light and DVD light are individuallyconstructed. Specifically, BD light is guided to the BD objective lens117 shown in FIG. 10B by the optical system for BD, and CD light and DVDlight are guided to the dual wavelength objective lens 116 by theoptical system for CD/DVD which is constructed independently of theoptical system for BD. The optical system for BD has a laser lightsource for emitting BD light, and one photodetector for receiving BDlight reflected on BD. The optical system for CD/DVD has a laser lightsource for emitting CD light and DVD light, and a photodetector otherthan the photodetector for BD light and for receiving CD light, DVDlight reflected on CD, DVD. The photodetector for CD/DVD has two sensorgroups for individually receiving CD light and DVD light. Similarly tothe inventive example, the optical system for BD is provided with ananamorphic lens for imparting astigmatism to BD light reflected on BD.The spectral element constituted of a multifaceted prism is disposed,for example, anterior to the anamorphic lens.

FIGS. 17A, 17B are schematic diagrams showing an arrangement of aspectral element 121 constituted of a multifaceted prism. FIG. 17A is aperspective view of the spectral element 121, and FIG. 17B is a planview of the spectral element 121 when viewed from an incident surfacethereof.

Referring to FIGS. 17A, 17B, the spectral element 121 is constituted ofa multifaceted prism. Surfaces 121 a through 121 d each inclined in adirection different from the optical axis of BD light are formed on theincident surface of the spectral element 121. BD light is entered intothe spectral element 121 in such a manner that the optical axis of BDlight is aligned with the center of the spectral element 121. With thisarrangement, BD light is uniformly entered into the surfaces 121 athrough 121 d. BD light entered into the surfaces 121 a through 121 d isrespectively refracted on the surfaces 121 a through 121 d in thedirections Va through Vd, and the propagating directions of BD light arechanged by the same angle. The directions Va through Vd coincide withthe directions Da through Dd shown in FIG. 4A, respectively.

Referring to FIG. 17B, the output surface of the spectral element 121 isformed with a light blocking portion 121 e having a width d and formedin parallel to the direction of a track image of entered light, and alight blocking portion 121 f having a width d and formed in a directionperpendicular to the direction of a track image of entered light. Thelight blocking portions 121 e, 121 f are formed by e.g. attaching alight blocking mask member on a flat output surface thereof. With thisarrangement, a part of BD light to be entered into the surfaces 121 athrough 121 d is blocked by the light blocking portions 121 e, 121 f.Specifically, BD light to be entered into areas 121 a 1, 121 a 2 withinthe surface 121 a, areas 121 b, 121 b 2 within the surface 121 b, areas121 c 1, 121 c 2 within the surface 121 c, areas 121 d 1, 121 d 2 withinthe surface 121 d is not blocked by the light blocking portions 121 e,121 f. As a result, the irradiation areas of signal light and straylight 1, 2 that have been transmitted through the spectral element 121are formed near the signal light area, as indicated by the irradiationareas a0 through h0 shown in FIGS. 13A through 13C. In this case, theirradiation areas a1 through h1 are not formed by light blocking by thelight blocking portions 121 e, 121 f.

In the modification example shown in FIGS. 17A, 17B, the surfaces 121 athrough 121 d are formed on an incident surface of the spectral element121, and the light blocking portions 121 e, 121 f are formed on a flatoutput surface of the spectral element 121. Alternatively, the surfaces121 a through 121 d may be formed on an output surface of the spectralelement 121, and the light blocking portions 121 e, 121 f may be formedon a flat incident surface of the spectral element 121. Furtheralternatively, a slope capable of obtaining refraction substantiallyequivalent to diffraction by the diffraction areas 118 a 1 through 118 h1 of the spectral element 118 may be formed, in place of the lightblocking portions 121 e, 121 f.

Further, in the inventive example, as shown in FIG. 11A, the verticallyoriented area and the transversely oriented area of the spectral element118 are configured to have the width d. Alternatively, the verticallyoriented area and the transversely oriented area of the spectral element118 may be configured to have widths different from each other.

FIG. 18A is a diagram showing an arrangement of the spectral element 118whose vertically oriented area and transversely oriented area havewidths different from each other. In the modified spectral element 118,the width d′ of the vertically oriented area is set smaller than thewidth d of the transversely oriented area. As shown in FIG. 18A, a lowerend of the diffraction areas 118 a 1, 118 h 1, and an upper end of thediffraction areas 118 d 1, 118 e 1 have a pointed-arrow shape.

FIG. 18B is a diagram showing light flux areas a0 through h0, a1 throughh1 of BD light that is entered into diffraction areas 118 a 0 through118 h 0, 118 a 1 through 118 h 1 of the modified spectral element 118.

As compared with the spectral element 118 shown in FIG. 11A, in the casewhere the modified spectral element 118 is used, the light flux areasa0, d0, e0, h0 are increased, and the light flux areas a1, d1, e1, h1are decreased. With this arrangement, when the focus position of BDlight is adjusted on a track of a target recording layer, theirradiation areas b0, c0, f0, g0 by the spectral element 118 shown inFIG. 18A are the same as those in the state shown in FIG. 13A, and theirradiation areas a0, d0, e0, h0 by the spectral element 118 shown inFIG. 18A are increased substantially by the same amount, as comparedwith the state shown in FIG. 13A. Further, a clearance to be formedbetween the irradiation areas b0, c0 in the above case, and a clearanceto be formed between the irradiation areas f0, g0 in the above case arethe same as those in the state shown in FIG. 13A; and a clearance to beformed between the irradiation areas a0, h0 in the above case, and aclearance to be formed between the irradiation areas d0, e0 in the abovecase are decreased, as compared with the state shown in FIG. 13A.

With the above arrangement, as shown in e.g. FIG. 14D, even in the casewhere the positions of the sensors B1 through B8 are displaced in adirection perpendicular to the direction of a track image, theirradiation areas on the sensors B1, B2, B7, B8 with less influence bythe displacement are set larger than those in the inventive example.Then, the precision of output signals from the sensors B1, B2, B7, B8are further enhanced as compared with the inventive example, whileenhancing the precision of output signals from the sensors B3 through B6in the same manner as the inventive example.

In the above modification example, since the width d′ of the verticallyoriented area is set smaller than the width d of the transverselyoriented area, the irradiation areas a0, d0, e0, h0 are set larger thanthose in the state shown in FIG. 13A substantially by the same amount.As a result, the output signals from the sensors B1, B2, B7, B8 are alsoincreased as compared with the state shown in FIG. 13A substantially bythe same amount. However, since the increased amounts of output signalsfrom the sensors B1, B2, B7, B8 are offset with each other in theequations (1), (2), there is no likelihood that such an increase mayaffect the focus error signal FE and the push-pull signal PP.Accordingly, even in the case where the spectral element 118 shown inFIG. 18A is used, it is possible to use the focus error signal FE andthe push-pull signal PP expressed by the equations (1), (2).

As described above, in the case where the sensors B1 through B8 arelikely to be greatly displaced in a direction perpendicular to thedirection of a track image resulting from e.g. aging deterioration, itis desirable to configure the spectral element 118 in such a manner thatthe width of the transversely oriented area is set larger than the widthof the vertically oriented area. Further, in the case where the sensorsB1 through B8 are likely to be greatly displaced in the direction of atrack image resulting from e.g. aging deterioration, it is desirable toconfigure the spectral element 118 in such a manner that the width ofthe vertically oriented area is set larger than the width of thetransversely oriented area. Furthermore, it is desirable to properly setthe widths of the vertically oriented area and the transversely orientedarea, in accordance with on a direction in which the sensors B1 throughB8 are likely to be displaced.

In the case where there is no or less likelihood that the positions ofthe sensors B1 through B8 may be displaced in the direction of a trackimage or in a direction perpendicular to the direction of a track image,the width of the transversely oriented area or of the verticallyoriented area may be set to zero.

FIG. 19A is a diagram showing an arrangement of a spectral element 122,in the case where the width of the vertically oriented area of thespectral element 118 is set to zero.

As shown in FIG. 19A, the spectral element 122 is formed withdiffraction areas 122 a through 122 h. As shown in FIG. 19A, an area (atransversely oriented area) formed by combining the diffraction areas122 g, 112 h has the width d. As shown in FIG. 19B, BD light passingthrough light flux areas a2 through h2 is respectively entered into thediffraction areas 122 a through 122 h of the spectral element 122. Thediffraction areas 122 a through 122 h diffract entered BD light in thesame manner as the diffraction areas 118 a 0, 118 b 0, 118 c 0, 118 d 0,118 f 0, 118 g 0, 118 b 1, 118 c 1 of the spectral element 118 in theinventive example.

In the above case, the irradiation areas by the light flux areas b2, c2,e2, f2 are respectively coincident with the irradiation areas b0, c0,f0, g0 shown in FIG. 13A. Further, signal light of BD light passingthrough the light flux areas a2, d2 is respectively irradiated ontoparts of the sensors B1, B2, and onto parts of the sensors B7, B8 in thesame manner as the case where the spectral element H is used.

With the above arrangement, as shown in e.g. FIG. 14D, even in the casewhere the positions of the sensors B1 through B8 are displaced in adirection perpendicular to the direction of a track image, the precisionof output signals from the sensors B3 through B6 are enhanced in thesame manner as the inventive example. Further, since the irradiationareas on the sensors B1, B2, B7, B8 are increased, the precision ofoutput signals from the sensors B1, B2, B7, B8 are further enhanced, ascompared with the inventive example.

Further, in the above case, output signals from the sensors B1, B2, B7,B8 are also increased substantially by the same amount, in the samemanner as the case where the spectral element 118 shown in FIG. 18A isused. However, the increased amounts of output signals are offset witheach other in the equations (1), (2), there is no likelihood that suchan increase may affect the focus error signal FE and the push-pullsignal PP. Thus, in this case, it is also possible to use the focuserror signal FE and the push-pull signal PP expressed by the equations(1), (2).

As described above, in the case where the positions of the sensors B1through B8 are greatly displaced only in a direction perpendicular tothe direction of a track image resulting from e.g. aging deterioration,it is desirable to configure the spectral element 118 in such a mannerthat the width of the vertically oriented area is set to zero. Further,in the case where the positions of the sensors B1 through B8 are greatlydisplaced only in the direction of a track image resulting from e.g.aging deterioration, it is desirable to configure the spectral element118 in such a manner that the width of the transversely oriented area isset to zero.

Furthermore, in the inventive example, the spectral element 118 isdisposed anterior to the anamorphic lens 119. Alternatively, thespectral element 118 may be disposed posterior to the anamorphic lens119, or a diffraction pattern for imparting the same diffractionfunction as the spectral element 118 to laser light may be integrallyformed on an incident surface or an output surface of the anamorphiclens 119.

The embodiment of the invention may be changed or modified in variousways as necessary, as far as such changes and modifications do notdepart from the scope of the claims of the invention hereinafterdefined.

1. An optical pickup device, comprising: a light source which emitslaser light; an objective lens which focuses the laser light on arecording layer; an astigmatism element which imparts astigmatism toreflected light of the laser light reflected on the recording layer; aspectral element into which the reflected light is entered, and whichseparates the reflected light; and a photodetector which receives thereflected light, wherein the astigmatism element converges the reflectedlight in a first direction and in a second direction perpendicular tothe first direction so that the reflected light forms focal lines atdifferent positions from each other, and the spectral element is dividedinto six second areas by a straight line in parallel to the firstdirection, a straight line in parallel to the second direction, and afirst area having a predetermined width and formed along a straight linein parallel to a third direction inclined from the first direction by 45degrees, and is configured to guide the reflected light passing throughthe six second areas to corresponding sensors on the photodetector whilemaking propagating directions of the reflected light different from eachother, and to avoid guiding the reflected light entered into the firstarea to the sensors.
 2. The optical pickup device according to claim 1,wherein the spectral element is divided into the eight second areas by athird area having a predetermined width and formed along a straight linein parallel to a fourth direction perpendicular to the third direction,in addition to the straight line in parallel to the first direction, thestraight line in parallel to the second direction, and the first area;and is configured to guide the reflected light passing through the eightsecond areas to the corresponding sensors on the photodetector whilemaking the propagating directions of the reflected light different fromeach other, and to avoid guiding the reflected light entered into thefirst area and into the third area to the sensors.
 3. The optical pickupdevice according to claim 2, wherein the spectral element changes thepropagating directions of the reflected light to be entered into thefirst area and into the third area in such a manner that an angle bywhich the first area changes the propagating direction of the reflectedlight and an angle by which the third area changes the propagatingdirection of the reflected light are set larger than an angle by whichthe second area changes the propagating direction of the reflectedlight.
 4. The optical pickup device according to claim 3, wherein thespectral element propagates the reflected light to be entered into eachof areas obtained by dividing the first area into two parts by thestraight line in parallel to the third direction, in directionsdifferent from each other, and propagates the reflected light to beentered into each of areas obtained by dividing the third area into twoparts by the straight line in parallel to the fourth direction, indirections different from each other.
 5. The optical pickup deviceaccording to claim 4, wherein the spectral element propagates thereflected light to be entered into each of the two areas obtained bydividing the first area by the straight line in parallel to the thirddirection, in directions in parallel to the first direction anddifferent from each other, and propagates the reflected light to beentered into each of the two areas obtained by dividing the third areaby the straight line in parallel to the fourth direction, in directionsin parallel to the second direction and different from each other. 6.The optical pickup device according to claim 2, wherein the first areaand the third area are so configured as to block the reflected light. 7.The optical pickup device according to claim 1, wherein the spectralelement guides the reflected light passing through the second area topositions of four different vertices of a rectangle, on a lightreceiving surface of the photodetector.